The known SBAS augmentation systems make it possible to deliver in real time corrections to the GNSS receivers in order, notably, to increase the accuracy of the geo-localization that is performed. They also make it possible to broadcast information used to improve the integrity of the service supplied by the system. Generally, the corrections and other information generated and broadcast by such systems are called augmentation data and are transmitted in the form of augmentation messages directly in the navigation signal.
To produce and broadcast such data the SBAS systems generally consist of RIMS ground stations which permanently measure the GNSS signals transmitted by the navigation satellites, processing centres which receive these measurements and generate the augmentation messages and NLES ground stations which transmit these messages by the GNSS navigation signal to an SBAS augmentation satellite which serves as a relay by retransmitting the received signal to the GNSS receivers. In such a system, the payload of an SBAS satellite is said to be transparent, which means that no processing resulting in a modification of the content of the user received signal is performed onboard the satellite.
Such systems have limitations regarding the availability and the continuity of operation that they offer which is guaranteed only at the cost of additional complexity of the system, notably through a redundancy of some equipment items.
Since the payload of an SBAS satellite is of transparent type, it does not allow simultaneous access to its resources. The implementation of hot redundancy between a nominal NLES ground station and a redundant standby NLES station is thus not possible because the SBAS satellite is capable only of receiving and retransmitting a signal at nominal power transmitted by a single NLES station called master or nominal station.
The expression hot redundancy is used with reference to a system for which at least two NLES ground stations can transmit a signal simultaneously over the uplink channel of the SBAS satellite. By contrast, the term cold redundancy is used when at least two stations are available for the transmission of the navigation signal to the SBAS satellite but they do not transmit simultaneously. The principle of cold redundancy is applied to the known SBAS systems. When a failure of the nominal NLES station is detected, the standby NLES station, which is not active by default, is started up in order to handle the switching of stations and the continuation of service. The time needed to start up the standby station results in a loss of continuity and of consequential service interruption, which may exceed a minute. This interruption time is also due to the following processing operations, which are necessary for re-establishing the link: detection of the fault, switch over to the redundant NLES station, stabilization of the station servo control loops, acquisition of the required integrity level.
Another problem associated with the transparent aspect of the SBAS satellite relates to the integrity of the navigation message received by this satellite. The known solutions implement an integrity check on the signals in the NLES ground stations. This check is performed by comparing the navigation signal transmitted on the uplink channel with the signal transmitted by the SBAS satellite on the downlink channel, which is picked up by the NLES stations.
The detection of a possible scrambling or misrepresentation of the navigation signals is performed on the ground and results either in the broadcasting of a specific alert message which is not instantaneous, or a cessation of transmission from the NLES station. There is no possible way of preventing the broadcasting of a misrepresented signal on the SBAS satellite itself, except by switching off the payload via a remote control link from the ground. This type of operation can lead to a loss of availability for the GNSS receivers which have to wait for the satellite to transmit a new valid signal.
A third problem lies in the use, for the transmission of the navigation signals transmitted by the NLES stations, of multiple frequency bands. For example, the GPS systems can use three frequency sub-bands in band L, namely the bands L1, L2 and L5 for various uses. Similarly, the European Galileo system provides for the use of four frequency sub-bands. Furthermore, the operational maintenance of the system may require the transmission of test channels.
The transmission of the signals over the uplink between a NLES station and the satellite is done conventionally according to an FDMA-type frequency plan and on a single polarization, that is to say that each signal is transmitted in the frequency band (L1, L2, L5, etc.) which corresponds to it. The multiplicity of the channels can therefore result in a very significant spectral occupancy, and lead to an increase in the complexity of the ground stations and of the payload.
Onboard the satellite, the payload provides a number of processing channels suited to each frequency band. The transmission of the signals on at least two distinct frequency bands leads to a differential dispersion in gain and phase between the navigation channels relative to these different frequencies. In practice, the propagation channel leads to different disturbances (noise, impact of the ionosphere) according to the transmission frequency. The signals must therefore be corrected in amplitude, in delay and in phase to compensate these differential errors. Furthermore, difficulties in pairing and calibrating the channels between them result in poor simultaneous management of signals transmitted on two or more distinct frequency bands, which results in a performance degradation for the user of the system.